Young players training less
I've trained Upamecano(20 years, 91 rated) a lot today(with boss coins) And since the last 2 trainings he has started to train much less and this is not the only case.
Why young players start to train less?
If you keep training same player sequentially, progression decreases.
For every training on same player and same simulation day, maximum training decrease. You can normally make 3 daily trainings per sim, but if you speed it up, you can do more, so it's natural that results decrease on every training!